1. Introduction
This invention relates to the chemical formation of protective, color receptive coatings on aluminum metals.
2. Description of the Prior Art
Methods for providing integral coatings on aluminum that are corrosion resistant, may be dyed, and act as bases for various finishes such as paint are known in the art. The most common method is anodizing which is an electrochemical method. Using anodizing procedures, a coating is formed on an aluminum part by passage of a current through an electrolyte such as sulphuric acid wherein the aluminum part is the anode and the tank is the cathode. Subsequent to formation of the coating, the part may be immersed in a dye bath to impart color to the coating. A process for anodizing aluminum is disclosed in the Metal Finishing Guidebook Directory for 1967, Metals and Plastics Publications, Inc., Westwood, New Jersey, pages 515 to 525.
Though anodizing is one of the most widely used methods for the oxidative treatment of aluminum, several disadvantages limit its efficiency. These include the high cost of anodization due to special equipment associated with electrolytic cells and power costs. Thus, electrodes, rectifiers, and specially lead lined or stainless steel tanks are required. Further, after prolonged use, the electrolyte must be discarded due to aluminum buildup.
To overcome some of these limitations, chemical methods for forming protective and color receptive aluminum surfaces have been suggested. The most widely used of these is disclosed in U.S. Pat. No. 2,976,371, incorporated herein by reference. In the process of said patent, there is disclosed an acidic treatment solution comprising a mixture of a chromic compound selected from the group of chromic acid and water soluble salts thereof and a ferricyanic acid and water soluble salts thereof. This treatment method effects a colorable complex of aluminum and chromic acid on immersion in said solution. This complex is capable of finishing such as by dyeing, painting or lacquering.
The above chemical means, while avoiding several of the disadvantages inherent in anodization, yields a soft coating that is dull in appearance, non-uniform, not heat resistant and which yields iridescent colors upon dyeing. Further, the chromic acid solutions present disposal problems.
In co-pending U.S. patent application Ser. No. 399,753, filed on Sept. 21, 1973, and in our U.S. Pat. No. 3,765,952, both incorporated herein by reference, a process for treating aluminum to provide a corrosion and heat resistant coating that is readily dyed is disclosed. In accordance with said process, an aluminum part is cleaned and desmutted as necessary and immersed in an aqueous alkaline solution of a ferricyanide compound for a time sufficient to treat the surface of the part. The treatment of the part with aqueous alkaline ferricyanide solution is believed to chemically form an oxide coating on the surface of the aluminum part similar to that formed electrolytically in an anodizing process. Subsequent to such treatment, the part may be dyed and/or finished as desired with for example suitable dyes, paints, and lacquers.
The aforesaid process overcomes the disadvantages noted above with respect both to anodizing and chemical treatment such as that disclosed in U.S. Pat. No. 2,976,371. Thus, the expensive electrical equipment and systems of the anodizing process are not needed. Yet, a coating is produced that is harder, more corrosion and heat resistant and receptive to more uniform, brighter and substantially more decorative finishes than that of prior chemical processes.